In my world, a Web Service is not a singular thing that runs on a web farm - it's ubiquituous in a huge infrastructure that's driven by Schema and WSDL. [Clemens Vasters: Enterprise Development & Alien Abductions]

I must admit that this was a blind spot in my thinking; I hadn't consdered the idea that there may exist WSDL contracts that are implemented by someone other than the owner. One might, for example, publish a WSDL contract that required callers to implement another contract where they receive callbacks. Consider the case in an industry where a standard WSDL interface is created, but two competitors implement extensions to the callback mechanism, without changing the namespace, causing chaos for a customer who needs to implement both endpoints. A simple, but imperfect solution would be to suggest the convention that namespace URIs should contain the domain name of the owner, and if you don't own the domain, you can't change the WSDL.

I think that there's broad agreement on the idea that published Schemas are immutable. This is a bit troubling because I know of some Schema versioning schemes that rely on a version attribute rather than the namespace to indicate version, which means that changing the value of gives one license to change the schema.